Cell membranes and passive transportEdexcel A-Level Biology B: Revision notes

The cell surface membrane is a phospholipid bilayer with hydrophilic phosphate heads facing the watery cytoplasm and outside, and hydrophobic fatty acid tails forming the core. The fluid mosaic model describes it: the phospholipids and proteins can move (fluid), and proteins are scattered through the bilayer (mosaic).

• Intrinsic (integral) proteins span the bilayer and form channel and carrier proteins.
• Extrinsic proteins lie on one surface, often acting as receptors or enzymes.
• Cholesterol sits between phospholipids and regulates fluidity and stability.
• Glycoproteins and glycolipids act in cell recognition and signalling.

How a molecule crosses the membrane depends on its properties:

• Small, non-polar, lipid-soluble molecules (oxygen, carbon dioxide, steroid hormones) dissolve in the bilayer and cross by simple diffusion.
• Polar molecules (glucose, amino acids) are too hydrophilic to cross the core and need carrier proteins.
• Charged ions (Na⁺, Cl⁻) cannot cross the hydrophobic core and need channel proteins.
• Water is polar but very small, and crosses the bilayer slowly by osmosis, faster through aquaporins.

Larger molecules cross more slowly.

Facilitated diffusion is the passive movement of polar molecules and ions through channel or carrier proteins, down a concentration gradient. It needs no ATP.

Its rate rises with the concentration gradient until all the carrier proteins are in use (saturated), when the number of carriers limits the rate. Simple diffusion has no such limit.

Osmosis is the net movement of water from a region of higher to lower water potential through a partially permeable membrane. Water potential (ψ) is measured in kPa, and pure water at atmospheric pressure is 0, so all solutions have negative values.

ψ = P + π, where P is pressure potential (turgor pressure, positive) and π is osmotic (solute) potential (negative).

In pure water a plant cell takes in water, becomes turgid and P rises until ψ of the cell equals 0 and there is no net movement. In a stronger solution water leaves, the cell becomes flaccid and then plasmolysed.

Beetroot cells contain red betalain in the vacuole. When membranes are damaged, betalain leaks out.

Method: cut discs of identical size from the same beetroot, rinse, and place equal numbers in equal volumes of distilled water at different temperatures for the same time. Measure the absorbance of the water with a colorimeter (blue-green filter).

Results: little leakage at low temperatures, a sharp rise above about 40 °C, when proteins denature and the bilayer becomes disrupted and more fluid.

Control: size of discs, volume of water, time, same beetroot. Improvements: repeats and means, a water bath, narrower temperature steps.

Plant tissue is placed in a range of sucrose solutions, and the percentage change in mass is calculated. Water enters if the solution has a higher water potential than the tissue (mass increases) and leaves if lower (mass decreases).

Method: cut cylinders of equal size, blot and weigh, leave for the same time in each solution, blot and reweigh. Plot percentage change in mass against concentration; the intercept on the axis is the concentration with the same water potential as the tissue. Convert to a water potential using a calibration of solutions.

Worked example: mass change +3.1% at 0.2 mol dm⁻³ and −4.2% at 0.4 mol dm⁻³ gives a zero point at about 0.28 mol dm⁻³.